In situ atomic-scale observation of oxygen-driven core-shell formation in Pt3Co nanoparticles

Sheng Dai(1), Yuan You(2,3), Shuyi Zhang(1,4), Wei Cai(4), Mingjie Xu(1,4), Lin Xie(1,5), Ruqian Wu(2), George W. Graham(1,4), Xiaoqing Pan(1,2), 2017

Image courtesy of Nature Communications

Abstract

The catalytic performance of core-shell platinum alloy nanoparticles is typically superior to that of pure platinum nanoparticles for the oxygen reduction reaction in fuel cell cathodes. Thorough understanding of core-shell formation is critical for atomic-scale design and control of the platinum shell, which is known to be the structural feature responsible for the enhancement. Here we reveal details of a counter-intuitive core-shell formation process in platinum-cobalt nanoparticles at elevated temperature under oxygen at atmospheric pressure, by using advanced in situ electron microscopy. Initial segregation of a thin platinum, rather than cobalt oxide, surface layer occurs concurrently with ordering of the intermetallic core, followed by the layer-by-layer growth of a platinum shell via Ostwald ripening during the oxygen annealing treatment. Calculations based on density functional theory demonstrate that this process follows an energetically favourable path. These findings are expected to be useful for the future design of structured platinum alloy nanocatalysts.

Impact Statement

Core-shell formation of platinum-alloy nanoparticles was observed in situ and it was discovered that the formation process is oddly oxygen-driven. It has been found previously that properties of the Pt shell are what drive the enhanced ORR activity and thus, the ability to both understand and more importantly control that shell is crucial to improving catalytic activity of these NPs. The authors both determine the mechanism of shell formation as well as develop a system to control that shell formation.